A steam turbine is used in power generation plants such as a nuclear power generation plant, a thermal power generation plant, a geothermal power generation plant. The steam turbine converts thermal energy of steam from high-pressure to low-pressure into a mechanical work. During this process, a temperature of the steam is lowered at a low-pressure part of the steam turbine, and a part of the steam condenses during an expansion work to increase wetness. The condensed moisture adheres or collides with a wall surface of a steam passage or a rotor blade of the steam turbine.
There is a case when the moisture adhered on the wall surface of the steam passage or the rotor blade grows into waterdroplet whose particle diameter is large. The waterdroplet of large particle diameter moves toward a rotor blade at a subsequent stage (downstream) by a flow of the steam. The waterdroplet of large particle diameter collides with a front edge and so on of the rotor blade at the subsequent stage, and erodes the rotor blade. Besides, the waterdroplet of large particle diameter generates a resistance for a rotation of the rotor blade (so-called a moisture loss). Namely, existence of moisture in the steam passage deteriorates turbine efficiency and reliability of the steam turbine.
Accordingly, in a conventional steam turbine, slits are provided at a stationary blade of a stationary component and a surface of a nozzle diaphragm outer ring of the stationary component supporting the stationary blade, and the moisture adhered on the surface of the stationary component of the steam passage is collected. The stationary component where the slits are provided has a hollow structure. The moisture collected by the slits pass through a hollow part, and is exhausted from the steam passage toward outside of the steam turbine.
The slits at the stationary blade and the surface of the nozzle diaphragm outer ring at the conventional steam turbine are formed to have a width of, for example, 1 mm or less. Accordingly, the slit is generally formed by electric discharge machining. However, there is a case when it is difficult to process the slit by the electric discharge machining because the nozzle diaphragm and so on become thick according to large-sizing of the steam turbine.